Difficulty: Moderately Easy
Step1
1. Put each keyword phrase into its own Ad Group.
This will allow you to easily pause, delete or adjust your bidding on a keyword phrase level. It makes it easy to manage your winning and losing keywords without going through the keyword lists and finding what to remove.
Step2
Use keyword tracking in your destination URL.
Most affiliate networks provide a way to add a tracking id to the affiliate link URL. For example, Clickbank calls it the "tid". So, when creating your hoplink, the screen has an input box to specify a tracking id.
By doing this, when you make a sale, clickbank show you which keyword made the sale.
Other affiliate networks use similar systems.
If you followed step 1, you can now increase your bid on the winning keywords and eliminate the keywords that are wasting your money.
Step3
Split test your Ads.
You can create ad variations for each Ad Group. Try creating 2 or 3 and Google will rotate them. You can then see which ones give you a higher CTR and then decide which ones to keep.